The present invention relates to improvements in microprocessor controlled transmission systems of the type employing both mechanical shifting of couplers and powershift clutch control to effect gear speed selection and having a creeper gear providing the ability to select extremely low output shaft to input shaft speeds. More particularly, the present invention relates to improvements in the control of engagement/disengagement of a creeper gear in response to operator control and also in response to short or open circuit conditions which may cause unexpected engagement or disengagement of the creeper gear.
The transmission described in the above-referenced copending applications has 16 forward and 16 reverse gears selectable by operation of two shift levers and four electro-hydraulically actuated powershift clutches. An optional creeper gear arrangement provides 8 addition very low speeds (forward or reverse) and is actuated by a manual switch which actuates a nonsynchronized electro-hydraulically actuated coupler. The creeper gear provides selection of extremely low speeds by increasing the ratio of input shaft speed to output shaft speed by a factor of about 5. Obviously, the creeper gear should be engaged or disengaged only under certain conditions to avoid unexpected changes in speed and gear clash on the non-synchronized creeper coupler. According to the present invention, the creeper gear mechanism is controlled so that it cannot be engaged/disengaged unless the operator has performed a sequence of steps indicating he is anticipating a speed change. The solenoid which controls the creeper gear arrangement is continuously monitored and if the current through the solenoid varies from pre-established norms while the creeper gear is engaged or not engaged, the creeper mechanism is disabled.